A resin foam refers to a low-density plastic containing a large number of interconnected pores or mutually independent pores which are uniformly distributed throughout the plastic. Pores in a foam are referred to as “foam cells.” Examples of commercialized foams include polystyrene foams, ABS foams, polyethylene foams, and polypropylene foams.
Foams are used for a variety of applications, depending on the property of the resin used. For example, they can be used for cushion materials, heat insulating materials, acoustic insulating materials, packing materials, electrical insulating materials, and light reflective materials. In particular, foams containing pores or small bubbles of the order of micrometers in size are used as visible light reflecting materials, exhibiting very high light reflectivity. With the conventional foaming methods, however, foam cells grow to 100 μm or more in size; therefore, most of the conventional methods can provide only foam sheets of over 1 mm in thickness. Moreover, it is difficult with these foaming methods to provide smooth sheet surface appearance. Large foam cells particularly lead to poor light reflection characteristics; resultant foam sheets fail to reach the practical level.
In addition to foams, stretched porous films containing inorganic or organic fillers have been also used as light reflective materials. However, films containing inorganic or organic fillers produce a large amount of residues when burned.
Methods of producing foams, particularly foams with fine foam cells, can be broadly classified into three types: batch process, injection molding, and continuous extrusion.
In a batch process (see, e.g., Patent Literatures 1-8), a solid resin composition is placed in a pressure-resistant container; a foaming agent is penetrated into the solid resin composition under high pressure; and the internal pressure of the container is rapidly reduced, allowing the foaming agent dissolved in the resin composition to become oversaturated for foaming to produce a foam, or the solid resin composition in which the foaming agent is penetrated is heated for foaming after taken it out as a solid from the container. The batch process has the advantage of capable of producing a foam having fine foam cells because a resin composition in solid state foams. However, as the batch process has the problem of limited depressurization rate, gas diffuses out of the film system before foaming occurs particularly where a thin film is used. It is thus difficult with the batch process to provide thin film foams.
Injection molding is a process in which a molten resin composition containing a foaming agent is injected into a mold cavity where it cools and hardens. While injection molding is advantageous in that cooling and hardening can be readily achieved by injection into a mold cavity, it is necessary to form an unfoamed skin layer on a molded product to provide good appearance. Specifically, a skin layer of unfoaming state is formed on the mold cavity surface without letting it foam by keeping high pressure in the mold cavity when a molten resin is injected in the mold cavity, and it is necessary to decrease pressure in the mold cavity by core back methods (the operation side of the mold is backed up to increase cavity volume in the mold) promptly afterwards and let the skin layer to foam.
Continuous extrusion (see e.g., Patent Literatures 9-11) is a process in which a resin mixture is plasticized and melted by an extruder and at the same time a foaming agent is dissolved in the resin mixture under high pressure and then the molten resin mixture is continuously extruded into a sheet or film by passing through a die, whereby the internal pressure of the extruder is instantly reduced back to atmosphere pressure. A foam is obtained in the same manner as that of the above batch process—the foaming agent dissolved in the molten resin composition is made to become oversaturated to foam. Continuous extrusion has the advantages of low production costs and easy mass production. In addition, continuous extrusion is capable of instant depressurization and therefore has the advantages of, for example, being capable of increasing the number of nucleation sites from which foam cells are produced, and of easily effecting foaming before the foaming agent dissolved therein is diffused and discharged to the outside.